The water-soluble ZnS quantum dots were prepared by co-precipitation method with sodium sulfide and zinc acetate as reactants and 3-Mercaptopropionic acid (MPA) as surface coated agent. The microstructure, morphology, size and optical properties of the products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and fluorescence spectrophotometer (PL). The results show that the products were irregular spherical cubic zinc blender ZnS with sizes around 4.8 nm and the emission maximized between 585 nm and 590 nm. The synthesis mechanism of ZnS quantum dots was studied by infrared spectroscopy(IR).